The general field of this invention is metal/zeolite catalysts. The invention is particularly concerned with catalysts which are formed by exchanging a Group VIII metal, such as platinum or nickel, onto zeolite, the highly dispersed form of the metal thus obtained being subsequently reduced to a zero valent catalytic state.
Catalysts formed from dispersions of catalytic metals, in zeolite supports have not been used as extensively as other catalysts for commercial purposes. Preferred catalytic supports for metals such as platinum and nickel or bimetallic combinations thereof have been alumina (Al.sub.2 O.sub.3) or silica (SiO.sub.2). Dispersions of catalytic metal on alumina or silica have been found to be much more stable in use than zeolite-supported catalysts. Under elevated temperature conditions as commonly encountered in the use of such catalysts, metal particles tend to migrate in the zeolite support, coalesce, and aggregate into larger particles with consequent loss of catalytic activity. See Imelik et al., "Catalysis by Zeolites," (1980, Elsevier, Amsterdam), pp. 235-249.
Since the first study of Pt supported in a calcium Y-type zeolite (CaY) by Rabo et al., Proc. 3rd Intern. Congr. Catal. (Amsterdam, 1964) 2:1264, metal-containing zeolites have been extensively studied. Most research has been done with Group VIII noble metals. The amine complexes of these cations are usually exchanged into the zeolites and reduced by H.sub.2. As an intermediate step, the amine complex may be treated in an oxydative atmosphere prior to the reduction with H.sub.2. It has been suggested that a low temperature (e.g. 300.degree. C.) for the metal reduction may be necessary to obtain highly dispersed metal particles entrapped in zeolite cavities. Dalla Betta, et al., 5th Intern. Congr. Catal. (Miami Beach, 1972) 1:329.
If metal ions on zeolite are reduced at high temperature, large metal particles may be formed having reduced catalytic activity. Even the highly dispersed metal obtained by low temperature reduction still tends to agglomerate to larger metal aggregates during use. The difficulties in obtaining and maintaining particle dispersions are indicative of a weak interaction of the metal particles with the zeolite framework. See Mortier, Proc. 6th Intern. Zeolite Conf. (Reno, 1983), Butterworths (1984), p. 734. Indeed it has been shown from molecular model calculations that the interaction is due only to weak van der Waals' forces. Sawr et al., Structure and Reactivity of Modified Zeolites (Elsevier, Amsterdam, 1984), p. 313.
It has been hypothesized that on alumina som catalytic metal remains in unreduced form, which may serve to anchor the reduced catalytically active metal. Huzinga and Prins, J. Phys. Chem. (1983), 87:173-176. However, in later work these authors did not find evidence confirming their earlier hypothesis. (Koningsberger et al., 8th Intern. Congr. Catal. (Berlin, 1984), V, 123).
Bimetal catalyst combinations on alumina and silica have been studied. Yermakov, Catal., Rev. Sci-Eng. (1976), 13:77-120. Improved catalyst stability was observed for platinum-rhenium-alumina and platinum-molybdenum-(or tungsten) silica. With reference to a Pt-Re-Al.sub.2 O.sub.3 catalyst, Yermakov assumed that only the the platinum was reduced, and that unreduced rhenium ions on the surfaces of the alumina support, in effect, provided anchors for the platinum.
Dispersing effects of metal ions in zeolite supported catalysts have been suggested. Exner et al., Chemie-Ing. Techn. (1980), 52:734-736. By comparing an iron-free synthetic zeolite with natural zeolite containing trace amounts of the iron impurity, it was concluded that the dispersion of platinum was improved by the presence of the iron impurity.